The present invention relates to a selective calling radio paging receiver in which the high power drain unit is rendered active at periodic intervals for power savings purposes and more specifically to a radio paging receiver having a verification circuit that verifies the validity of a received digital signal in the event of a loss or corruption of a preamble.
In conventional battery saving radio pagers, the high power drain unit is rendered active for a duration sufficient to permit the receiver to search for a preamble that precedes a subsequent digital signal and rendered inactive for an interval much longer than the active duration if the preamble is not detected. If the preamble is detected, the battery saving operation is disabled to permit the receiver to search for a synchronization codeword to establish word synchrozniation and for a match between a received address and the user's address to energize a tone generator for alerting the user. However, if a preamble is lost or corrupted prior to the reception of a subsequent signal, the receiver recognizes it as an absence of paging signal and the subsequent signal will be lost.
A Power saving radio pager, as shown and described in U.S. Pat. No. 4,523,332 issued to T. Mori, overcomes this problem by monitoring the bit rate of a digital signal received during its short active period and verifying it as a valid signal if it has a predetermined bit rate. Specifically, the pager employs a counter for counting binary level transitions of received digital pulses and a noise detector that clears the counter to the initial value whenever a noise impulse occurs in the received bit sequence. Although the noise component must be excluded from the count if it is desired to ensure a high degree of verification, the occurrence of a single noise impulse near the end of a count sequence would nullify the count which could be used as a valid indication of a paging signal. Therefore, the prior art paging receiver is likely to be affected by noise impulses which can be tolerated for most practical applications.